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通过Gleeble-3500热模拟试验机研究了不同应变速率下V-N微合金化Q420B钢连铸坯的高温热塑性,利用扫描电镜观察高塑性区和第Ⅲ脆性温度区拉伸试样的断口形貌及断口处组织形貌,分析了试验钢在高温下的强度和塑性随温度变化的关系,动态再结晶、相变和析出物对高温热塑性的影响。结果表明:在应变速率为ε觶=5×10-3/s时,存在第Ⅲ脆性区(700~900℃),在1000℃时断面收缩率(RA)达到最大值92.16%;当应变速率为ε觶=5×10-2/s时,存在第Ⅲ脆性区(600~862℃),在1100℃时RA达到最大值90.39%;当应变速率为ε觶=5×10-1/s时,不存在塑性凹槽;3个应变速率下均没有出现第Ⅱ脆性区;在第Ⅲ脆性区,随着应变速率的增大,断面收缩率提高;在1000~1200℃出现高塑性的主要原因是发生了动态再结晶;第Ⅲ脆性区塑性低主要是由于晶界处有析出物和夹杂物,同时也是由于沿奥氏体晶界析出的铁素体抗拉强度低。
The thermal plasticity of VN microalloyed Q420B steel billets at different strain rates was investigated by Gleeble-3500 thermal simulator. The fracture morphology and fracture of the tensile specimens in the high plasticity zone and the third brittle temperature zone were observed by scanning electron microscopy The microstructure and morphology of the test steel were analyzed. The relationship between strength and plasticity of the test steel at high temperature and the effect of dynamic recrystallization, phase transformation and precipitation on the high temperature thermoplasticity were analyzed. The results show that the third brittle zone (700-900 ℃) exists at the strain rate of ε 觯 = 5 × 10-3 / s and the maximum value of the cross section shrinkage (RA) reaches 92.16% at 1000 ℃. When the strain rate For ε 觯 = 5 × 10-2 / s, the third brittle zone (600 ~ 862 ℃) exists and reaches the maximum value of 90.39% at 1100 ℃. When the strain rate is ε 觯 = 5 × 10-1 / s , There is no plastic groove; no second brittle zone appears at all of the three strain rates; in the third brittle zone, the reduction of area decreases with the increase of strain rate; the main plasticity at 1000-1200 ℃ The reason is the occurrence of dynamic recrystallization. The low plasticity of the third brittle zone is mainly due to the precipitates and inclusions in the grain boundaries, and also the low tensile strength of ferrite precipitated along the austenite grain boundaries.